Type physicalism (also known as Type Identity Theory, Type-Type theory or just Identity Theory) is the theory, in the philosophy of mind, which asserts that mental events are type-identical to the physical events in the brain with which they are correlated. It is called type identity in order to distinguish it from a similar but distinct theory called the token identity theory.

The type/token distinction is easily illustrated by way of example. In the phrase "yellow is yellow is yellow is yellow", there are only two types of words ("yellow" and "is") but there are seven tokens (four of one and three of the other). The thesis of type physicalism consists in the idea that mental event types (e.g. pain in all individual organisms of all species at all times) are, at least contingently, identical with specific event types in the brain (e.g. C-fibre firings in all individual organisms of all species and at all times).

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According to Ullin Place, one of the popularizers of the idea of type-identity in the 1950s and '60s, the idea of type-identical mind/body physicalism originated in the 1930s with the psychologist E. G. Boring and took nearly a quarter of a century to finally catch on and become accepted by the philosophical community. Boring, in a book entitled The Physical Dimensions of Consciousness (1933) wrote that:

To the author a perfect correlation is identity. Two events that always occur together at the same time in the same place, without any temporal or spatial differentiation at all, are not two events but the same event. The mind-body correlations as formulated at present, do not admit of spatial correlation, so they reduce to matters of simple correlation in time. The need for identification is no less urgent in this case.

The barrier to the acceptance of any such vision of the mind, according to Place, was that philosophers and logicians had not yet taken a substantial interest in questions of identity and referential identification in general. The dominant epistemology of the logical positivists at that time was phenomenalism, in the guise of the theory of sense-data. Indeed Boring himself subscribed to the phenomenalist creed, attempting to reconcile it with an identity theory and this resulted in a reductio ad absurdum of the identity theory, since brain states would have turned out, on this analysis, to be identical to colors, shapes, tones and other sensory experiences.

The revival of interest in the work of Gottlob Frege and his ideas of sense and reference on the part of Herbert Feigl and J.J.C. Smart, along with the discrediting of phenomenalism through the influence of the later Wittgenstein and J.L. Austin, led to a more tolerant climate toward physicalistic and realist ideas. Logical behaviorism emerged as a serious contender to take the place of the Cartesian "ghost in the machine" and, although not lasting very long as a dominant position on the mind/body problem, its elimination of the whole realm of internal mental events was strongly influential in the formation and acceptance of the thesis of type identity.

There were actually subtle but interesting differences between the three most widely credited formulations of the type-identity thesis, those of Place, Feigl and Smart which were published in several articles in the late 1950s. Place's notion of the relation of identity was derived from Bertrand Russell's distinction among several types of is statements: the is of identity, the is of equality and the is of composition. Place's version of the relation of identity is more accurately described as a relation of composition. For Place, higher-level mental events are composed out of lower-level physical events and will eventually be analytically reduced to these. So, to the objection that "sensations" do not mean the same thing as "mental processes", Place could simply reply with the example that "lightning" does not mean the same thing as "electrical dicharge" since we determine that something is lightning by looking and seeing it, whereas we determine that something is an electrical discharge through experimentation and testing. Nevertheless, "lightning is an electrical discharge" is true since the one is composed of the other.

For Feigl and Smart, on the other hand, the identity was to be interpreted as the identity between the referents of two descriptions (senses) which referred to the same thing, as in "the morning star" and "the evening star" both referring to Venus. So to the objection about the lack of equality of meaning between "sensation" and "brain process", their response was to invoke this Fregean distinction: "sensations" and "brain" processes do indeed mean different things but they refer to the same physical phenomenon. Moreover, "sensations are brain processes" is a contingent, not a necessary, identity.

One of the most influential and common objections to the type identity theory is the argument from multiple realizability. The multiple realizability thesis asserts that particular mental events can, and likely are, implemented differently in different species of organism (and also within single organisms at different times) and therefore the idea that, e.g., "pain is identical to C-fiber firings" universally and at all times is highly implausible. The idea of token identity (that only particular occurrences of mental events are identical with occurrences or tokenings of physical events) and functionalism both strongly entail a commitment to multiple realizability. The response of type identity theorists such as Smart to this objection is that, while it may be true that mental events are multiply realizable, this does not demonstrate the falsity of type identity. As Smart states it:

"The functionalist second order [causal] state is a state of having some first order state or other which causes or is caused by the behavior to which the functionalist alludes. In this way we have a second order type theory."

The fundamental point is that it is extremely difficult to determine where, on the continuum of first order processes, type identity ends and merely token identities begin. Take Quine's example of English country gardens. In such gardens, the tops of hedges are cut into various shapes, for example the shape of an elf. We can make generalizations over the type elf-shaped hedge only if we abstract away from the concrete details of the individual twigs and branches of each hedge. So, whether we say that two things are of the same type or are tokens of the same type because of subtle differences is just a matter of descriptive abstraction. The type-token distinction is not all or nothing.

Hilary Putnam responds to Smart's response by essentially rejecting functionalism because, he believes, it is indeed a second-order type identity theory. Putnam uses mutiple realizability against functionalism itself, suggesting that mental events (or kinds, in Putnamian terminology) may be diversely implemented by diverse functional/computational kinds; there may be only a token identification between particular mental kinds and particular functional kinds. Putnam, and many others who have followed him, now tend to identify themselves as generically non-reductive physicalists. Putnam's invocation of multiple realizabiility does not, of course, directly answer the problem posted by Smart with respect to useful generalizations over types and the flexible nature of the type-token distinction in relation to causal taxonomis in science. But see multiple realizability for a more in-depth discussion of these issues.

Another frequent objection is that type identity theories fail to account for phenomenal properties (or qualia). These are those properties of mental events (having a pain, feeling sad, experiencing nausea) which are generally considered introspective and non-reducible to brain properties. The type identity theorist, such as Smart, attempts to explain away such phenomena by insisting that the experiential properties of mental events are topic-neutral. The concept of topic-neutral terms and expressions goes back to Gilbert Ryle, who identified such topic-neutral terms as "if", "or", "not", "because" and "and." If one were to hear these
terms alone in the course of a coversation, it would be impossible to tell whether the topic under discussion concerned geology, physics, history, gardening or selling pizza. For the identity theorist, sense data and qualia are not real things in the brain (or the physical world in general) but are more like "the average electrician." The average electrician can be further analysed and explained in terms of real electricians but is not itself a real electrician.